For a method of seismic retrofitting for existing architectural structures, such as a building or a condominium, a method of reinforcing columns and beams inside of a building or of adding an anti-seismic wall is available. Such a retrofitting method, however, is not favorable enough because it requires the work inside of the building, meaning that the building cannot be used during the work.
Then another method of providing the outer wall surfaces of an existing building with seismic retrofitting has been mainstream so as to enable the seismic retrofitting while allowing the use of the existing building, and typical examples thereof include a method of direct-attaching a framed steel brace and a method of adding a framed steel brace structure.
The method of direct-attaching a framed steel brace is to directly attach a framed steel brace internally including a steel brace to the outer wall surfaces of an existing building. This method is not suitable for an outer wall surface provided with an overhang such as a balcony, eaves or a louver, because the steel brace and the overhang interfere with each other.
The method of adding a framed steel brace structure is to construct the foundation specific to a steel brace structure beside the outer wall surface to be reinforced, and a steel brace structure is added one by one on this foundation. Referring now to FIG. 11, this method of adding a framed steel brace structure is described in details.
As shown in FIG. 11, a foundation K including an underground beam not illustrated is firstly added to the left and right outer wall surfaces along the longitudinal direction of an existing building B, such as a condominium, and this underground beam is connected to the underground foundation of the existing building B for integration. Thereafter, a steel brace structure H is constructed on the foundation K to the top story while joining outer columns of the existing building B and outer beams on each floor with the steel brace structure H for seismic retrofitting.
FIG. 12 shows various types of cross-sectional forces generated at the joints of the steel brace structure H and the existing building B.
In FIG. 12, Meh denotes bending moment at the joint, Quh denotes a shear force at the joint, and Ne denotes a tensile force at the joint, and the relationships of Meh=Quh×eh and Ne=Meh/L hold, where QP denotes a shear force at the added structure that is Quh, eh denotes a distance between the steel brace core and the beam end, and L denotes a width of the steel brace structure H viewed from the front.
As shown in the drawing, a horizontal shear force only is transmitted between the added steel brace structure H and the existing building B, while a vertical shear force is transmitted to the added foundation K via the vertical members of the retrofitted steel brace structure H, and therefore the foundation K has to be added. Further, a tensile force Ne is generated at the joint between the steel brace structure H and the existing building B, which results from eccentric bending moment.
In this way the foundation K has to be added, and therefore also when seismic retrofitting just for the middle-level floors or the top-level floors is to be performed, the foundation K has to be added, and a steel brace structure H standing from the foundation K, i.e., the steel brace structure H including practically unnecessary steel braces for lower-level floors, has to be constructed. This necessitates such an uneconomical retrofitting structure, and since the foundation K to be added has to be within the clearance limit, if it is difficult to construct the foundation, such retrofitting strategy cannot be performed.
Instead of the steel brace structure H, another seismic retrofitting structure is available as shown in FIG. 13, which provides a stud-type dumper D between the outer beams OB on any upper-level and lower-level floors of an existing building. Such a stud-type dumper D is fixed to the outer beams OB using anchor bolts A via base plates P, and a large drawing force X will act on the anchor bolts A, which results from bending moment generated similarly to FIGS. 11 and 12 (a pressing force X will act on the anchor bolts on the other side). In order to act against this drawing force X, through holes have to be bored in the outer beams OB, and tendons TB, such as PC steel rods, have to be disposed therein and be tightened for joining. If the building does not have such outer beams OB, outer beams OB have to be added so as to act against the drawing force X at the tendons TB.
For the conventionally known techniques, Patent Documents 1 and 2 are available. Patent Document 1 describes the technique of providing an existing building with a seismic retrofitting frame having a retrofitting post and a retrofitting steel beam externally, in which the retrofitting steel beam is joined to an existing outer beam without joining the retrofitting post and an existing outer post. This structure makes a horizontal force generated during earthquakes act on the seismic retrofitting frame, and therefore the existing building can have a seismic retrofitted structure. Such a structure, however, still has the problem as stated above because the retrofitting post has to be constructed from the foundation, meaning that the foundation specific to the seismic retrofitting frame is required.
Patent Document 2 describes the technique of forming a pin supporting portion at a post-beam joint on the outer surface of an existing building, supporting an outer shell reinforcing frame including an outer shell post frame that is elongated upward and downward from each layer so that the outer shell post frame, an outer shell beam frame that is elongated continuously along the beams, and the pin supporting portion make up the post-beam joint, and making a connection at the gap between the outer shell post frames elongated upward or downward, thus constructing a lattice outer shell reinforcing frame on the outer surface of the existing building.
Such an outer shell reinforcing structure does not require the addition of the foundation for the outer shell reinforcing structure, but the structure simply includes the pin supporting portions at the post-beam joints on the outer surface of the existing building, and therefore it is not certain whether, if a large drawing force acts on the pin supporting portions as stated above, the strength of these joints can resist the drawing force or not.